ACQUISITIONS AND DISPOSITIONS - Disposals (Details)												3 Months Ended		9 Months Ended
	Oct. 17, 2017 USD ($)	Sep. 26, 2017 USD ($) apartment	Sep. 21, 2017 USD ($) ft²	Aug. 31, 2017 USD ($) ft²	Jun. 23, 2017 USD ($) apartment	Jun. 20, 2017 USD ($) ft²	Jun. 08, 2017 USD ($) ft²	May 30, 2017 USD ($) apartment	Mar. 28, 2017 USD ($) ft²	Jul. 19, 2016 USD ($) room	Feb. 02, 2016 USD ($) room	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) business	Sep. 30, 2016 USD ($) business	Dec. 31, 2016 USD ($)
Disposals
Number of business acquisitions | business														0	0
Disposed Property Information
Sales price														$ 851,629,000	$ 94,568,000
Gain on sale of real estate												$ 74,715,000	$ 14,927,000	378,732,000	39,666,000
Assets
Total assets												173,897,000		173,897,000		$ 0
Liabilities
Total liabilities												36,948,000		36,948,000		$ 0
Additional Information
Impairment of real estate												0	$ 0	13,100,000	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Assets
Investments in real estate, net												469,816,000		469,816,000
Deferred rent receivable and charges, net												29,954,000		29,954,000
Other intangible assets, net												11,283,000		11,283,000
Other assets												38,000		38,000
Total assets												511,091,000		511,091,000
Liabilities
Debt, net												86,477,000		86,477,000
Other liabilities												14,029,000		14,029,000
Intangible liabilities, net												1,800,000		1,800,000
Total liabilities												102,306,000		102,306,000
Additional Information
Premium on assumed mortgage												665,000		665,000
Disposal Group, Held-for-sale, Not Discontinued Operations
Assets
Investments in real estate, net												167,361,000		167,361,000
Cash and cash equivalents												1,720,000		1,720,000
Accounts receivable, net												543,000		543,000
Deferred rent receivable and charges, net												4,105,000		4,105,000
Other assets												168,000		168,000
Total assets												173,897,000		173,897,000
Liabilities
Debt, net												28,618,000		28,618,000
Accounts payable and accrued expenses												6,980,000		6,980,000
Due to related parties												323,000		323,000
Other liabilities												1,027,000		1,027,000
Total liabilities												36,948,000		36,948,000
Additional Information
Investments in real estate, gross												217,304,000		217,304,000
Investments in real estate, accumulated depreciation												49,943,000		49,943,000
211 Main Street, San Francisco | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage									100.00%
Sale transaction costs									$ 2,943,000
Mortgage prepayment penalty									$ 1,508,000
Disposed Property Information
Square feet | ft²									417,266
Sales price									$ 292,882,000
Gain on sale of real estate									$ 187,734,000
3636 McKinney Avenue and 3839 McKinney Avenue, Dallas, Texas | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage								100.00%
Sale transaction costs								$ 2,258,000
Mortgage prepayment penalty								$ 1,901,000
3636 McKinney Avenue, Dallas, Texas | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposed Property Information
Number of units | apartment								103
Sales price								$ 20,000,000
Gain on sale of real estate								$ 5,488,000
3839 McKinney Avenue, Dallas, Texas | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposed Property Information
Number of units | apartment								75
Sales price								$ 14,100,000
Gain on sale of real estate								$ 4,224,000
200 S College Street, Charlotte, North Carolina | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage							100.00%
Sale transaction costs							$ 833,000
Disposed Property Information
Square feet | ft²							567,865
Sales price							$ 148,500,000
Gain on sale of real estate							$ 45,906,000
980 9th Street and 1010 8th Street, Sacramento, California | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage						100.00%
Sale transaction costs						$ 1,119,000
Disposed Property Information
Square feet | ft²						485,926
Sales price						$ 120,500,000
Gain on sale of real estate						$ 34,559,000
4649 Cole Avenue, Dallas, Texas | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage					100.00%
Sale transaction costs					$ 3,311,000
Mortgage prepayment penalty					$ 2,812,000
Disposed Property Information
Number of units | apartment					334
Sales price					$ 64,000,000
Gain on sale of real estate					$ 25,836,000
800 N Capitol Street, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage				100.00%
Sale transaction costs				$ 2,388,000
Disposed Property Information
Square feet | ft²				311,593
Sales price				$ 119,750,000
Gain on sale of real estate				$ 34,759,000
7083 Hollywood Boulevard, Los Angeles, California | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage			100.00%
Sale transaction costs			$ 584,000
Disposed Property Information
Square feet | ft²			82,193
Sales price			$ 42,300,000
Gain on sale of real estate			$ 23,670,000
47 E 34th Street, New York, New York | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage		100.00%
Sale transaction costs		$ 3,157,000
Disposed Property Information
Number of units | apartment		110
Sales price		$ 80,000,000
Gain on sale of real estate		$ 16,556,000
Courtyard Oakland, Oakland, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage											100.00%
Disposed Property Information
Number of rooms | room											162
Sales price											$ 43,800,000
Gain on sale of real estate											$ 24,739,000
LAX Holiday Inn, Los Angeles, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Disposals
Ownership sold, percentage										100.00%
Disposed Property Information
Number of rooms | room										405
Sales price										$ 52,500,000
Gain on sale of real estate										$ 14,927,000
370 L'Enfant Promenade, Washington D.C.
Additional Information
Impairment of real estate												0		13,100,000
4200 Scotland Street, Houston, TX
Additional Information
Outstanding principal balance												28,715,000		28,715,000
4200 Scotland Street, Houston, TX | Disposal Group, Held-for-sale, Not Discontinued Operations
Disposed Property Information
Sales price														64,025,000
Additional Information
Deferred loan costs												264,000		264,000
Deferred loan costs, accumulated amortization												$ 167,000		$ 167,000
Subsequent Event | 370 L'Enfant Promenade, Washington D.C. | Disposal Group, Held-for-sale, Not Discontinued Operations
Disposals
Ownership sold, percentage	100.00%
Disposed Property Information
Sales price	$ 126,680,000
Additional Information
Gain on sale of real estate property	$ 2,000,000